[vm/bytecode] Keep unreachable code for code coverage

This change adds a bytecode generation option 'keep-unreachable-code' to
avoid eliminating unreachable code from constant conditions.
This option is enabled in kernel service.

If unreachable code is eliminated, then call instructions are not generated
and ICData objects are not created. Source code report is collected
using created ICData objects and will not flag the unreachable code as
not executed.

Change-Id: Ic73e19c653505e0dfab4648ab65ef6b90370e0b4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/113960
Reviewed-by: Régis Crelier <regis@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

pkg/vm/bin/kernel_service.dart

@@ -158,26 +158,30 @@ abstract class Compiler {
       if (options.bytecode && errors.isEmpty) {
         await runWithFrontEndCompilerContext(script, options, component, () {
           // TODO(alexmarkov): disable source positions, local variables info,
-          //  debugger stops and source files in VM PRODUCT mode.
+          //  debugger stops, source files and unreachable code in VM PRODUCT
+          //  mode.
           // TODO(rmacnak): disable annotations if mirrors are not enabled.
           generateBytecode(component,
               coreTypes: compilerResult.coreTypes,
               hierarchy: compilerResult.classHierarchy,
               options: new BytecodeOptions(
-                  enableAsserts: enableAsserts,
-                  environmentDefines: options.environmentDefines,
-                  // Needed both for stack traces and the debugger.
-                  emitSourcePositions: true,
-                  // Only needed when the debugger is available.
-                  emitLocalVarInfo: true,
-                  // Only needed when the debugger is available.
-                  emitDebuggerStops: true,
-                  // Only needed when the VM service is available.
-                  emitSourceFiles: true,
-                  // Only needed when reload is available.
-                  emitInstanceFieldInitializers: true,
-                  // Only needed when mirrors are available.
-                  emitAnnotations: true));
+                enableAsserts: enableAsserts,
+                environmentDefines: options.environmentDefines,
+                // Needed both for stack traces and the debugger.
+                emitSourcePositions: true,
+                // Only needed when the debugger is available.
+                emitLocalVarInfo: true,
+                // Only needed when the debugger is available.
+                emitDebuggerStops: true,
+                // Only needed when the VM service is available.
+                emitSourceFiles: true,
+                // Only needed when reload is available.
+                emitInstanceFieldInitializers: true,
+                // Only needed when mirrors are available.
+                emitAnnotations: true,
+                // Only needed when observatory (source report) is available.
+                keepUnreachableCode: true,
+              ));
           component = createFreshComponentWithBytecode(component);
         });
       }

pkg/vm/lib/bytecode/gen_bytecode.dart

@@ -1290,6 +1290,9 @@ class BytecodeGenerator extends RecursiveVisitor<Null> {
   /// Returns value of the given expression if it is a bool constant.
   /// Otherwise, returns `null`.
   bool _constantConditionValue(Expression condition) {
+    if (options.keepUnreachableCode) {
+      return null;
+    }
     // TODO(dartbug.com/34585): use constant evaluator to evaluate
     // expressions in a non-constant context.
     if (condition is Not) {

pkg/vm/lib/bytecode/options.dart

@@ -13,6 +13,8 @@ class BytecodeOptions {
     'show-bytecode-size-stat': 'Show bytecode size breakdown',
     'source-positions': 'Emit source positions',
     'instance-field-initializers': 'Emit separate instance field initializers',
+    'keep-unreachable-code':
+        'Do not remove unreachable code (useful if collecting code coverage)',
   };
 
   bool enableAsserts;
@@ -24,6 +26,7 @@ class BytecodeOptions {
   bool emitAnnotations;
   bool emitInstanceFieldInitializers;
   bool omitAssertSourcePositions;
+  bool keepUnreachableCode;
   bool showBytecodeSizeStatistics;
   Map<String, String> environmentDefines;
 
@@ -37,6 +40,7 @@ class BytecodeOptions {
       this.emitAnnotations = false,
       this.emitInstanceFieldInitializers = false,
       this.omitAssertSourcePositions = false,
+      this.keepUnreachableCode = false,
       this.showBytecodeSizeStatistics = false,
       this.environmentDefines = const <String, String>{}}) {
     causalAsyncStacks ??=
@@ -64,6 +68,9 @@ class BytecodeOptions {
         case 'instance-field-initializers':
           emitInstanceFieldInitializers = true;
           break;
+        case 'keep-unreachable-code':
+          keepUnreachableCode = true;
+          break;
         case 'show-bytecode-size-stat':
           showBytecodeSizeStatistics = true;
           break;